Create or update leads in bulk

Creates or updates up to 1,000 leads in one request. Leads are matched by email within the workspace. If a lead with that email already exists, it is updated: a field you send with a value is changed, and a field you leave out, or send as null or an empty string, is left as it is. To clear a field, use PUT /leads/{id} with an empty string. middleName is only saved when a lead is created. customVariables, when sent with at least one variable, replaces the lead's whole set; leave it out to keep the set. Every lead needs a valid email: one invalid lead fails the whole request with 400, and nothing is saved. To add the leads to a campaign, pass campaignId at the top level of the request body, not inside a lead. The campaign must have at least one email account attached (POST /campaigns/{id}/sender-emails). If it has none, the leads are saved and added to the campaign, but the call fails with 500; attach an account and send the same request again. count in the response is the number of leads you sent, not the number created. Any attribute outside the named fields can be sent in customVariables and becomes a merge tag usable in email copy. PUT /leads is an alias for this endpoint and behaves identically.